The present invention relates to high-frequency, high-power resonant converters, and more particularly, to a novel resonant power converter having a plurality of power transformers with insured current sharing therebetween.
It is highly desirable to provide DC--DC power converters with high efficiency and high power density, which typically requires that a relatively high switching frequency be utilized. The use of high switching frequencies, typically on the order of 100-500 Khz., is relatively easy to attain with relatively low power converters, but is relatively difficult to obtain when a single high-power transformer is driven. This is so because, as the output power required of a power converter is increased, the size and the leakage inductance of the output transformer increases. The output transformer volume varies with the output power in accordance with the relationship (V1/V2)=(P1/P2).sup.3/4, where V1 or V2 is the respective transformer volume at respective first or second power level P 1 or P2; the transformer leakage inductance varies substantially linearly with dimension and, therefore, with power rating. From the foregoing relationships, along with the fact that volume varies as the cube of dimension, one can derive that transformer leakage inductance L, as a function of power rating P, is given by (L1/L2)=( P1/P2).sup.174 and where L2 are the respective leakage inductances for transformers having respective power ratings P1 and P2. Because the resonant converter output voltage must be reduced by an amount proportional to leakage inductance and output current, the available output voltage will decrease as the converter power rating is increased, if all other factors are held equal. To maintain the same output power, additional voltage and current stresses on the solid state components will be required, if the same frequency is maintained. In practice, therefore, the frequency is usually lowered as the power rating of the converter is increased, to offset the effect of the larger transformer, while simultaneously reducing high frequency losses in the larger transformer conductors. Additionally, higher output current has often been achieved by paralleling diodes, introducing current sharing and thermal runaway problems in the diodes as well as the necessity for selecting diodes with matched characteristics. It is highly desirable to provide a resonant power converter in which a high operating frequency is maintained even while the power level increases, and to allow mismatched secondary rectifiers to be utilized for current sharing in such increased-power-rating resonant power converters.